Electrochemical fuel cells convert fuel and an oxidant to electricity and one or more reaction products.
Solid polymer fuel cells of the PEM type generally include a membrane electrode assembly (“MEA”) layer comprising a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers (a cathode and an anode). The electrode layers, or gas diffusion layers (GDLs), typically comprise porous, electrically conductive sheet material and an electrocatalyst at each membrane/electrode interface to promote the desired electrochemical reaction.
In PEM fuel cells employing hydrogen and oxygen as the reactant gases (hydrogen as the fuel and oxygen as the oxidant), the reaction product is water. The reactions are as follows:Anode: H2→2H++2e−Cathode: ½O2+2H++2e−→H2OAt the anode, the fuel (hydrogen) moves through the porous electrode material and is oxidized at the anode electrocatalyst to form cations that migrate through the membrane to the cathode. At the cathode, the oxidizing gas or oxidant (typically oxygen contained in air) moves through the porous electrode material and is reduced by reaction with the cations at the cathode electrocatalyst to form the reaction product (water).
For optimal operation, each electrode must be provided with an even and continuous supply of the respective reactant (fuel or oxidant). During operation, the proton exchange membrane in the MEA of a PEM fuel cell must be saturated with water to reach its full electricity production performance and to minimize its electrical resistance. As set out above, reactant-product water is produced at the cathode of the fuel cell. The distribution of water within the MEA is affected by the operation of the fuel cell. Some of the water produced at the cathode tends to diffuse through the proton exchange membrane toward the anode. However, ion drag, the tendency of the ions moving in the proton exchange membrane to drag water molecules with them, tends to cause water in the proton exchange membrane to move toward the cathode. One measure of a fuel cell efficiency is its electrical current output per electrode area, referred to as current density At current densities above approximately 1 amp/cm2, ion-drag-caused movement of water within the proton exchange membrane dominates the movement of water due to normal diffusion, and water tends to collect at the cathode side of the MEA. As current densities increase, the amount of water produced in a given electrode area also increases. Both adequate provision of reactant gases and saturation of the proton exchange membrane require proper water management, viz removal of excess reaction-product water when and where such water is preventing a reactant gas from contacting or readily passing through an electrode; and provision of water when and where the proton exchange membrane is not saturated.
When the oxidant is oxygen in an airflow, ensuring an even and continuous supply of oxidant is complicated by the fact that oxygen makes up only a relatively small portion of the gases in air. As air contacts the cathode, oxygen is consumed by the fuel-cell reaction; therefore, the oxygen concentration in the air tends to decrease the longer the air is in contact with the electrode. As well, the fuel-cell reaction is exothermic. PEM fuel cells operate most effectively at temperatures well above normal ambient temperatures, usually around 85 C, and the fuel cell design must accommodate the dissipation of excess heat.
In conventional fuel cells, the MEA layer is interposed between two substantially fluid-impermeable, electrically-conductive plates, commonly referred to as separator plates. The separator plates serve as current conductors; provide structural support for the electrode layers; and function as key elements for passing waste heat to heat rejecting elements. As well, the separator plates typically provide distribution means for directing the fuel and oxidant to the anode and cathode layers, respectively; and for exhausting reaction products. Typically, these distribution means are channels embossed or machined on each surface of a separator plate that abuts an electrode. In use, the fuel, oxidant and reaction-product water flow through the channels. The channels on a separator plate are collectively referred to as a flowfield. Separator plates having such channels are sometimes referred to as fluid flowfield plates. In use, the channels deliver reactant gases to the electrodes, and the porous electrode materials allow the reactants to diffuse from the channels to the surface of the ion exchange membrane where catalyst materials are located to encourage the reaction. The conductive porous electrode material and the flowfield plates conduct electric current produced at the MEA. Good electrical contact between the flowfield plate and the electrode is important for minimizing electrical resistance within a fuel cell.
Reactant (fuel and oxidant) distribution, electrical contact between the fluid flowfield plates and the electrodes, water management, and oxygen consumption are all affected by the design of the fluid flowfield, including (i) the ratio of channel and land width (the lands separate the channels and abut the adjacent electrode plate, thus establishing electrical contact between the flowfield plate and the electrode plate); (ii) the number and length of channels; (iii) the depth of the channels; and (iv) the general pattern of the flowfield.
Even distribution of reactant gases and even distribution of electrical contact with a fluid flowfield plate are normally achieved by configuring the flowfield to have a relatively large number of small channels and narrow lands rather than a smaller number of larger channels and wider lands. A preferred combination of channel and land width that optimizes the distribution/diffusion of reactant gases and electrical conductivity is often determined through extensive analysis and experiment.
As oxidant air flows along a channel, the concentration of oxygen in the air decreases as the air moves from the upstream to the downstream end of the channel. If the flow in the channel is essentially laminar, the oxygen concentration of the air flowing directly over the surface of the cathode may be less than the oxygen concentration of the rest of the air in the channel. Therefore, it is generally desirable to mix the air as it flows in the channel to avoid a performance-decreasing reduction in the oxygen concentration at the surface of the cathode. It has been found that bends, and rounded corners in the channels tend to induce and promote mixing. However, although mixing of the air in a channel is generally desirable, turbulent flow of the air in a channel is generally not desirable, as turbulent flow may cause eddies or dead spots where water and/or air depleted of oxygen may collect.
Keeping the proton exchange membrane saturated and removing excess water requires a balancing of competing requirements in the flowfield design, and typically requires humidification of the reactant gases (or one of the two) before they enter the fuel cell. For example, unless humidified, ambient reactant air traveling through a channel tends to enter in a relatively dry state, since even humid ambient air will have a relatively low relative humidity once it is heated to the typical fuel-cell operating temperature of about 85 C. As the air moves downstream in a channel, it tends to pick up reaction-product-water vapor, and the relative humidity of the air rises. Therefore, as the air moves downstream, the relative humidity of the air may rise to the point of saturation, at which the air will cease to be a net absorber of water vapor from the MEA. Hence, the tendency of the air to dry the MEA is greatest near a channel inlet and typically decreases to zero as the air moves downstream in the channel. As water is produced all along the length of the channel by the fuel-cell reaction and the air flowing in the channel has a decreasing tendency to absorb water vapor as it moves along the channel, excess water typically tends to accumulate in the downstream portion of a channel. Excess water may accumulate and/or condense inside the reaction flow-channels as well as in the pores of the electrode, a condition known as flooding. Flooding impedes the passage of the reactant gas to and/or through the electrode, reducing fuel cell performance. When water starts to accumulate or condense inside one of the reactant gas channels, the gas flow rate (and velocity) in the channel decreases, and depending on the flowfield pattern and the reactant gas feed means, the flow of reactant gas may be simply diverted to the remaining channels. This diversion reduces the flow in the partially clogged channel and reduces the water-propulsive effect of the reactant gas in the channel, which typically results in a build-up of water within the channel and the GDL pores adjacent to it, until the channel is fully clogged by water. The electrochemical reaction at the affected local active area of the fuel cell will either stop or considerably decrease. The diverted reactant gas flow may also worsen the flow conditions of other flow-channels, and more flow-channels may become clogged one after another. As water continues to accumulate within the flow-channels, the overall performance of the fuel cell may drop significantly due to the gradual reduction of its effective active area.
The known flowfield designs attempt to optimize reactant (fuel and oxidant) distribution; electrical contact between the separator plates and the electrodes; water management; and oxygen consumption. Different general categories of patterns of flowfield channels are known, including: single-channel serpentine flowfields; multiple-parallel-channel flowfields; and multiple-channel serpentine flowfields. Single-channel serpentine flowfields typically comprise a channel with multiple straight parallel legs with 180° switchback turns at each end connecting each leg to the adjacent legs on either side. Such a single-channel serpentine flowfield is disclosed in U.S. Pat. No. 4,988,583 (issued 29 Jan. 1991 to Watkins et al.). Multiple-parallel-channel flowfields typically comprise a set of straight parallel channels that run across the face of the separator plate. Multiple-channel serpentine flowfields typically comprise multiple channels of substantially equal length each having multiple straight legs of different lengths. Such a multiple-channel serpentine flowfield is disclosed in U.S. Pat. No. 5,108,849 (issued 28 Apr. 1992 to Watkins et al.).
Single-channel serpentine flowfields as a class are relatively effective for removing excess water from the channel. In use, the channel in a single-channel serpentine flowfield typically has a relatively large pressure drop along its length because of its small area and long length, and thus water is effectively propelled along the channel. However, the increased pressure required to propel the reactant and excess water through the length of the channel in a single-channel serpentine flowfield tends to have a high parasitic power requirement as compared to other flowfield patterns, and therefore typically results (despite any improved water-removal capability) in lower overall system efficiency. A high flow rate can also induce drying (particularly near the inlet where little reactant water exists). As well, the relatively long length of the channel in a single-channel serpentine flowfield may result in oxygen depletion in the portion of the channel proximate to the downstream end of the channel outlet. Such oxygen depletion may result in an inadequate supply of oxidant to the cathode and thus poor fuel-cell performance.
As compared to single-channel serpentine flowfields, the problem of oxygen depletion is minimized with multiple-parallel-channel flowfields, because of the relatively-shorter channels in the multiple-parallel-channel flowfields. As well, it may be possible to orient multiple-parallel-channel-flowfield fuel cells such that gravity assists in propelling excess water from the channels. However, gravity is typically not completely effective in propelling excess water, and multiple-parallel-channel flowfields generally require a relatively-high flow rate in each channel to purge water. Water causing flooding or clogging in one or a few channels may not be effectively purged since the reactant gas can simply divert through the unflooded channels. Typically, reactant pressure and flow rates in a multiple-parallel-channel flowfield must be significantly higher than with a single channel flowfield, causing a significantly higher parasitic load and dehydration problems in the upstream portions of the channels in a multiple-parallel-channel flowfield. As well, typically, the oxidant air is not effectively utilized since it resides in the channel for only a short time before exiting the fuel cell.
Generally, the best-performing combination of number of channels and bends lies somewhere between single-channel serpentine flowfields and multiple-parallel-channel flowfields, and therefore the multiple-channel serpentine flowfield is the current industry standard. However, although multiple-channel serpentine flowfields typically perform better than single-channel serpentine flowfields and multiple-parallel-channel flowfields, multiple-channel serpentine flowfield fuel cells typically require a relatively high reactant flow rate (with associated high parasitic load) and/or means for balancing flow between channels, to effectively propel excess water from the channels.
U.S. Pat. No. 5,776,625 (issued 7 Jul. 1998 to Kaufman et al.) discloses a multiple-channel serpentine flowfield pattern in which the upstream portion of each channel is adjacent to the downstream portion of another channel (or the same channel in the case of single-channel serpentine flowfields). This so-called counter-current-flow configuration is intended to mitigate the drying effect of the reactant gases in the upstream portions of the channels by locating the upstream portion of a channel adjacent to a region of the MEA having a plentiful supply of water (the downstream portion of an adjacent channel) so that water will diffuse to the region being dried by the reactant gases. Kaufman discloses restrictions at the inlets of the all flow channels within the fuel cell stack intended to ensure uniform air flow among all of the channels. Kaufman does not deal with the removal of excess water.
U.S. Pat. No. 6,387,558 (issued 14 May 2002 to Mizuno et al.) describes the addition of an interconnecting manifold at the mid point of serpentine flow fields to improve performance by balancing flow rates within various channels. Mizuno does not deal with the removal of excess water.
U.S. Pat. No. 6,099,984 (issued 8 Aug. 2000 to Rock) describes the flow that can occur between channels (or legs of the same channel) in a flowfield when one channel is at a higher pressure than the other. When such a pressure differential exists, the reactant gas will tend to flow through the porous electrode, bypassing the land separating the channels (or legs of the same channel). Rock teaches that such cross-flow may be advantageous in that it may permit gas flow between adjacent legs of the same channel even when the flow channel is blocked (e.g. by a water droplet). A disadvantage of such cross flow with some flowfield patterns is that if a channel outlet is adjacent a channel inlet, the higher-pressure at the channel inlet may cause some of the reactant gas to cross flow to the channel outlet, thereby bypassing the desired flow path through the channel. Rock discloses a flowfield pattern intended to overcome this disadvantage. Rock does not deal with the removal of excess water.
What is needed is an improved means for managing water in fuel cells so as to reduce flooding and water clogging of the channels and porous electrodes.